GREASE SELECTION STEPS
MAJOR PARAMETERS OF SELECTION
Base Oil Viscosity Base Oil Group Additives NLGI Number (Consistency) Thickener Performance Properties
BASE OIL VISCOSITY Speed Factor : Speed factors account for the surface speed of the bearing elements and are determined by the following formula: Speed Factor = NDm Where, N = rpm and Dm = ( bearing bore + outside diameter) / 2 [UoM is mm] The NDm value uses pitch diameter rather than bore diameter because not all bearings of a given bore have the same element diameter, and thus have different surface speeds. Knowing the speed factor value and likely operating temperature, the minimum viscosity requirement can be read directly from charts in the following figure ......
As it is clear from the fi gure, 1) we will have to go for higher VG, if temperature increases for same speed factor. 2) We will have go for lesser VG if speed factor increases temperature remaining same.
LET US MAKE IT SIMPLER FOR OUR OPERATING CONDITIONS : DmN Factor
K.V. at 40 C
Below 10000 > 10000 > 20000 > 30000 > 50000 > 60000 > 80000 100000 and above
460 (cSt) 320 220 150 100 68 32 10/ 15
°
BASE OIL TYPE Most greases are formulated using API Group I and II mineral oil base stocks, which are appropriate for most applications. synthetic base oils are required for : • high or low operating temperatures • a wide ambient temperature range • any application where extended re-lubrication intervals are desired.
ADDITIVES Once the appropriate viscosity has been determined, it’s time to consider additives. Most performance-enhancing additives found in lubricating oils are also used in grease formulation and should be chosen according to the demands of the application. Figure 2 shows some common additive requirements by application :
Journal Bearings
Ball Bearings
Thrust Bearings
Roller Bearings
Needle Bearings
Antioxidants
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Antifoam Agents
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•
•
•
•
•
•
•
•
•
•
•
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•
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Additive
Antiwear/EP Rust Inhibitors Extreme Pressure Demulsibility VI Improvers Corrosion Inhibitors
• • • Required,
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•
•
Depends on application
•
GREASE CONSISTENCY Now for that extra step: The consistency of grease is controlled by the thickener concentration, thickener type and the viscosity of the base oil. Even though base oil viscosity affects consistency, it is important to note that a grease can have a high consistency and a low base oil viscosity or vice versa. (Higher viscosity do not necessarily lead to higher consistency) The NLGI has established a scale to indicate grease consistency which ranges from grades 000 (semifl uid) to 6 (block grease). The most common NLGI grade is 2 and is recommended for most applications. For bearings, speed factor and operating temperature can be used to determine the NLGI grade for a given application. Following table provides a general guide to selecting NLGI grade based on speed factor and operating temperature.
Operating Temperature
DN (Speed Factor)
NLGI No.*
-30 to 100 F (-34 to 38 °C)
0 - 75,000 75,000 - 150,000 150,000 - 300,000
1 2 2
0 to 150 F (-18 to 66 °C)
0 - 75,000 75,000 - 150,000 150,000 - 300,000
2 2 3
100 to 275 F (38 to 135 °C)
0 - 75,000 75,000 - 150,000 150,000 - 300,000
2 3 3
°
°
°
* Depends on other factors as well, including bearing type, thickener type, base oil viscosity and base oil type As can be observed above, for most applications, NLGI grade 2 is applicable.
THICKENER Numerous types of grease thickeners are currently in use, each with its own pros and cons :
Thickener Soap type
Simple Soap Li - base Ca - base Na - base Al - base
Non soap Poly urea, clay pigments etc. Complex Soap Li - complex Al - complex Ca – complex Ti – complex Ca-Sulphonate
PERFORMANCE PROPERTIES Once the appropriate base oil viscosity, additive requirements and consistency have been determined, the remaini ng criteria to consider are the performance properties. Grease performance properties include many of the same properties used for lubricating oils Properties exclusive to grease include : • Dropping point • Mechanical stabil ity • Water washout • Bleed characteristics • Pumpability.
ANOTHER IMPORTANT FACTOR IS
QUANTUM OF GREASE USED
N/ Ng
Fill in bearing house space (%)
Upto 0.2 0.2 To 0.5 0.5 To 0.8 0.8 To 1.0
80 50 30 10
N = Actual RPM Ng = Max. Permissible RPM
Re-lubrication Quanti ty G = OD x B / 200 gm OD = Bearing Outer Diameter in mm B = Width of inner race bearing in mm
